The present invention relates generally to power distribution systems and, more particularly, to a system and method for automatically detecting and localizing high resistance ground faults (HRGFs) in a high resistance grounded (HRG) power distribution system, with a pulsing current utilized for localizing the HRGFs being automatically initiated and terminated.
A ground fault is an undesirable condition in an electrical system in which electrical current flows to the ground. A ground fault occurs when the electrical current in a distribution or transmission network leaks outside of its intended flow path. Distribution and transmission networks are generally protected against faults in such a way that a faulty component or transmission line is automatically disconnected with the aid of an associated circuit breaker. As one particular example, an HRG power distribution system limits fault current (typically to less than 5 amperes) in order to minimize downtime of the system, with the system remaining in service as long as only a single ground fault is present. However, with such HRG power distribution systems, it is recognized that locating the ground fault in a timely manner is required in order for downtime minimization to be realized (i.e., location of the ground fault while only a single ground fault is present).
As a means to provide for locating a ground fault in an HRG power distribution system, some recent HRG power distribution systems have been configured as HRG pulsing systems—with such systems making use of a pulsing current that is introduced into the system in order to provide for locating the ground fault. One such example of an HRG power system that utilizes test signals or pulses to trace an HRGF is set forth in U.S. Pat. No. 7,180,300 to General Electric Co., in which processors are used to calculate relationships between current and voltage phase angles present in a power distribution system, with the technique reading the current and voltage, calculating the zero sequence current (after subtracting the capacitive charging current), then running this signal through a low pass analog filter in order to determine a change in the RMS amplitude value of the zero sequence current before and after pulsing—with a faulted feeder being identified if the magnitude of the output of the filter exceeds some pre-determined value. The technique thus provides for an automated means for locating a ground fault in an HRG power distribution system, albeit with the drawbacks that the technique is very complex and computationally intensive (while at the same time, certain elements are not robust in being able to detect a fault) and that the technique requires the use of voltage sensors due to the need for extra sensitivity to differentiate between the capacitive charging current and the actual pulsing ground current, therefore adding cost to the system.
Another example of an HRG power distribution system that utilizes an HRG pulsing system to locate a ground fault is described in U.S. application Ser. No. 14/291,161, filed May 30, 2014, by Eaton Corporation and which is hereby incorporated by reference in its entirety. As described therein, and as reproduced herebelow in FIG. 1, the HRG power distribution system 10 includes a power transformer 12 having an input side 14 and an output side 16. The power transformer 12 comprises three phases, i.e., a first phase 18, a second phase 20, and a third phase 22 that are coupled, in the example of FIG. 1, per the angle of the primary and secondary windings. That is, the third phase 22 on the primary has the same angle as what is shown as the first phase 18 on the secondary. Likewise, the first phase 18 on the primary is coupled with the second phase 20 shown on the secondary, and the second phase 20 on the primary is coupled with what is shown as the third phase 22 on the secondary.
The three phases 18, 20, 22 of the power transformer 12 are coupled to a plurality of three-phase distribution networks 24, 26. A load 28, such as an induction motor, for example, is connected to each distribution network 24, 26 to receive three phase power therefrom. Each distribution network 24, 26 is also provided with a circuit breaker 30.
The HRG power distribution system 10 includes a neutral line 32 at the output side 16 of the power transformer 12 that is grounded via one or more grounding resistors 34 included in the HRG pulsing system 36. The grounding resistors 34 are configured to reduce the ground fault current, so that the HRG power distribution system 10 can remain in operation while a ground fault is being located. That is, when there is an occurrence of a ground fault in the HRG power distribution system 10, the grounding resistors 34 limit the ground fault current.
The HRG pulsing system 36 also includes a ground fault sensor 37 that senses an occurrence of a ground fault in the HRG power distribution system 10 and signals a test signal generator 38 (i.e., “pulsing circuit”) that is incorporated into the HRG pulsing system 36 and is configured to introduce a test signal into the HRG power distribution system 10. The pulsing circuit 38 includes a pulsing switch or contactor 40 and associated controller 42 provided to generate a pulsing current 44 in the HRG power distribution system 10. One of the grounding resistors 34 is periodically partially shorted or, alternatively, one of the ground resistors 34 is connected in addition to the other ground resistor 34 by closing the pulsing contactor 40 (via controller 42) to generate the pulsing current 44 at desired intervals.
As further shown in FIG. 1, a ground fault locating system 48 is provided for the HRG power distribution system 10. The ground fault locating system 48 includes a plurality of current sensors 50, 52 coupled to the three-phase HRG power distribution system 10, for measuring values of the instantaneous three-phase current. The current sensors 50, 52 are positioned on respective distribution networks 24, 26 and are located on the distribution networks to measure three-phase current signals at a respective protection device 54 connected thereto. The protection devices 54 may be in the form of protection relay units included in the ground fault locating system 48. The protection relay units 54 operate as highly configurable motor, load, and line protection devices with power monitoring, diagnostics and flexible communications capabilities—including controlling contactors 56 on the distribution networks 24, 26. The current signals generated/measured by current sensors 50, 52 are provided to processors 58 that are incorporated into the protection relay units 54, with the processors 58 performing computations for identifying the presence of a ground fault indicative of an HRGF condition in the HRG power distribution system 10.
After activation of the HRG pulsing system 36 and during the time where a ground fault exists on a system, the HRG pulsing system 36 modulates (pulses) the ground current between two distinct values (e.g., between 5 and 10 amperes on a 600V class or less low voltage system). The ground fault locating system 48 is capable of detecting zero-sequence (ground) current to determine if the pulsing current 44 is present (as shown in the distribution network 24) or absent (as shown in the distribution network 26). If one of the protection relay units 54 detects the pulsing current 44, that motor protection relay unit is located somewhere between the point of the ground fault and the incoming source. With portable ground sensing equipment, isolation of the ground fault occurs when one of the sensing units 54 is positioned just downstream from the point of the ground fault, at which point the pulsing current 44 cannot be detected by the sensing units 54. This transition from detecting the pulsing current 44 to no longer detecting the pulsing current 44 is the exact location of the ground fault. In permanently mounted ground sensing equipment—where the position of the ground current cannot be moved—the location of the ground will be isolated to be somewhere between a downstream sensing unit 54 that detects no fault and upstream sensing unit 54 that does detect a fault.
However, in each of the HRG power distribution systems set forth in U.S. Pat. No. 7,180,300 and U.S. application Ser. No. 14/291,161, introduction and termination of the pulsing current 44 is presently performed via a manual controlling of the HRG pulsing system 36. While this manual introduction and termination of a pulsing current is not a limitation or drawback in HRG power distribution systems where locating the ground fault is done with a hand-held ammeter—i.e., since an operator is still needed to carry the portable current sensing device and read and interpret the pulsing signature of the current, such that manually controlling the pulse current is not an appreciable additional burden—it is recognized that the manual controlling of the pulsing current/test signal in automated HRGF detection/localization systems is less than ideal. That is, as the HRG power distribution system incorporates means for automatically detecting a pulsing zero-sequence current signature and localizing a ground fault (thus eliminating the need for manual localization), requiring an operator to manually introduce and terminate the pulsing current becomes an unnecessary step that unduly delays the process of detecting and locating a ground fault, with such manual activation of the HRG pulsing system making it impossible to localize the ground fault within an optimal timeframe after it appears.
Furthermore, it is recognized that removal/termination of the pulsing current as soon as is practical after localization of the ground fault is also highly desirable. That is, leaving the HRG pulsing system on for an extended time beyond that which is necessary in order to localize the ground fault within the HRG power distribution system increases the wear not only on the device(s) switching the current (e.g., pulsing contactor 40), but also on the insulation of the HRG power distribution system, as generation of the pulsing current results in transient voltage that can be modeled as V=L*di/dt (with V being voltage, L being inductance of the series path carrying the current, and di/dt being the instantaneous rate of current change), where the dt term is small and the voltage term V is large. While existing systems rely on an operator to manually deactivate/terminate the pulsing current upon localizing of the ground fault, it is recognized that even the most skilled operators may forget that the pulsing current is active and/or may be called away from the HRG power distribution system, resulting in leaving the HRG pulsing system on for an extended time. Even when an operator acts in an appropriate fashion to terminate the pulsing current as soon as is practical, there will always be a delay between when the ground fault is localized and when the operator will be able to access the HRG pulsing system to switch off the pulsing current, and this time can vary based on operator efficiency and accessibility of the HRG pulsing system.
It would therefore be desirable to provide a system and method that provides an automated approach for introducing and terminating a pulsing current used for detecting and localizing an HRGF in a three-phase power distribution system, so as to allow for localization of a ground fault within an optimal timeframe and prevent unnecessary wear on pulsing contactors and insulation in the HRG power system. The system and method would also beneficially eliminate the need for manual introduction and termination of the pulsing current and locating of the ground fault (e.g., with a hand-held ammeter) so as to reduce safety concerns and costs associated with such manual activation, termination and detection.